Copper Complexes of New Redox‐Active 4,5‐Bisguanidino‐Substituted Benzodioxole Ligands: Control of the Electronic Structure by Counter‐Ligands, Solvent, and Temperature

et al. 2017

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A new redox-active 4,5-bisguanidino-substituted o-benzoquinone ligand L is synthesized, which allows rational access to heterobinuclear complexes through the sequential coordination of two metals. In the examples discussed in this work, mononuclear Cu and Pd complexes are prepared in a first coordination step, and these complexes are then used as precursors to homobinuclear [Cu -L -Cu ] and heterobinuclear [Pd -L -Cu ] complexes. In the heterobinuclear complex, the Pd is coordinated by the softer bisguanidine side of L and the Cu by the harder dioxolene side (in line with the HSAB concept). The heterobinuclear complex is in a temperature-dependent equilibrium with its dimer, with two unsymmetrical Cu-Cl-Cu bridges. The redox-chemistry of the [Cu -L-Cu ] and [Pd -L-Cu ] complexes is studied. One-electron oxidation of both complexes was found to be quasi-reversible in CV experiments, and chemical one-electron oxidation was achieved with NO (SbF ). In the case of the homobinuclear complex [L(CuCl ) ] , intramolecular ligand-metal electron-transfer, triggered by coordination of a CH CN solvent molecule, leads to a temperature-dependent equilibrium between the form [Cu -L -Cu ] at low temperatures (with CH CN coordinated to the Cu atom) and [Cu -L -Cu ] at higher temperatures (without CH CN).

“…[25] Cu(OH) 2 (85 wt %) and CuCl 2 was purchased from Sigma Aldrich and PdCl 2 (MeCN) 2 from Strem. All solvents were dried rigorously prior to their use.…”

Section: Methodsmentioning

confidence: 99%

“…Precursor compound 3 can be synthesized as reported in our previous work. [25] Cu(OH) 2 (85 wt %) and CuCl 2 was purchased from Sigma Aldrich and PdCl 2 (MeCN) 2 from Strem. The Supporting Information contains information concerning the applied analytical techniques and the DFT computations.…”

Section: Methodsmentioning

confidence: 99%

Homo‐ and Heterobinuclear Cu and Pd Complexes with a Bridging Redox‐Active Bisguanidino‐Substituted Dioxolene Ligand: Electronic Structure and Metal–Ligand Electron‐Transfer

Schrempp

Schneider

et al. 2017

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“…In this aspect, the results of this study are in line with the results on valence isomerism found for some copper-guanidine complexes. [19] The direct coordination of solventm olecules does not play ar ole because the electronic and steric properties of guanidine-copper complexes do not favor coordination numberslarger than four. [23] HATand radicalrecombination Withins everalh ours, the initially red CH 3 CN solutionst urn colorless and the formerly green CH 2 Cl 2 solutions adopt ag rey- blue color (see Supporting Information, Figures S3, S4, S17, S18).…”

Section: Electron-transfers Tepmentioning

confidence: 99%

Hydrogen‐Atom Transfer (HAT) Initiated by Intramolecular Ligand–Metal Electron Transfer

Herrmann

Himmel

2017

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Hydrogen-atom transfer (HAT) is of key importance for several catalytic and biological processes, and provides an elegant access to C-H activation. In synthetic chemistry, a photoactivated metal complex is often employed to abstract an oxygen- or nitrogen-bound hydrogen, and the as-generated oxygen- or nitrogen-centered radical is the hydrogen-atom acceptor for HAT. Here, we report the first examples for HAT processes initiated by one-electron oxidation of urea azines. A further novelty is that the HAT-initiating oxidation can be realized by intramolecular ligand-metal electron transfer in copper(II)-urea azine complexes. These complexes are first characterized in the solid state, in which they are stable. Electron-transfer-initiated HAT processes are observed upon dissolving the complexes in organic solvents, and the kinetics of these processes varies with the solvent polarity. The carbon-centered radicals formed by HAT can either be trapped with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) or undergo radical recombination reactions with itself, yielding diamagnetic end-products.

“…[36,38,39]), to achievet he optimal balance between fast activation of dioxygen with an efficient phenol deprotonation and the ability to oxidizel ess electron-rich phenols, is currently under development. The expansion of this catalytic system to redox-activeg uanidine ligands with slightly higher redox-potential (see the bisguanidine ligands in refs.…”

Section: Discussionmentioning

confidence: 99%

“…Based on these results, the oxidative cross-coupling reactiono fp henolsw ith a non-complementary relationship is studied in detail. Hence it was possible to shift electrons between the redox-active guanidine ligand and the coppera tom in ac omplex by changing the counterions, [35] the solvent [36][37][38] or the temperature. This ligand, which is oxidized reversibly at low potential( E 1/2 = À0.7 Vv s. Fc + /Fc in CH 2 Cl 2 solution for the redox couple 1 2 + + /1), [34] supplies the metal atom with extra electrondensity,u pt ot he point of full transfer of two electrons from the ligand to the two coppera toms.…”

Section: Introductionmentioning

confidence: 99%

Catalytic Aerobic Phenol Homo‐ and Cross‐Coupling Reactions with Copper Complexes Bearing Redox‐Active Guanidine Ligands

Schön

Himmel

2019

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Due to their large importance in synthetic chemistry,c atalytic CÀCc oupling reactions of phenolsa re currently intensively studied. Herein, new copperc atalysts for the CÀC coupling reactiono fp henols using dioxygen as ag reen oxidizing reagent are reported. By using redox-active guanidine ligands, the activity as well as chemoselectivity in the cross-coupling reaction of non-complementary phenols( between an electron-rich phenol and al ess nucleophilic second phenol)i ss ignificantly improved. Based on the collected data for severalt est reactions,areaction mechanism is proposed.Scheme1.Flow-chartfor generalp rinciples in the phenol cross-coupling reactions. Adapted from Pappo et al. [20] [a] F.